Fiberglassing a horizontal surface

Hi.  I searched for the answer to my question and was unable to find it...  My car is a 1990 Mercedes 300ce (coupe version of the e-class).  For those unfamiliar with the early-90's E class cars, the trunk is sealed off from the cabin because the gas tank is mounted vertically just behind the rear seats.  Since the trunk is so sealed up, high fidelity bass cannot be had without some modifications.

My subwoofer installation requires me to cut a hole in the rear dash and mount the sub inside the cabin firing straight up (just like the rear speakers).  I want to use a sealed enclosure.  The underside of the rear dash (i.e. the metal surface inside the trunk) is not exactly flat--there are grooves and various other angles to it.  I would like to use fiberglass since the enclosure will be suspended by the rear dash (i.e. "hanging" from it).  So MDF is too heavy.  I also want to use fiberglass because it will allow me to mold into the non-flat underside of the rear dash.

So, now that you all have an understanding of what I'm trying to do: how do I go about fiberglassing a horizontal surface?  I have read about people having trouble because they can't get the FG matting material to stick to the first layer of resin.  I can see how this could be a problem for me, especially since the area is probably 3.5' x 2'.  Would I be able to pop the mold out only after putting down a single layer of resin (i.e. no fiberglass)?

I've never used fiberglass before, so I'm not sure how tacky/sticky the resin mixture gets.  Thanks very much for your help.

That's great advice.  Thank you very much!  It sounds like this is quite a production, but I think it will be worth it.

Follow-up question- I know the normal process is to apply a coat of resin first, let dry for a while, apply one layer of FG, wait, apply more FG, etc etc.  If I apply 3 coats at the same time to the tinfoil/Foam/MDF apparatus and compress it against the underside of the rear dash, how will this affect drying time?  Could i maybe just do one layer of FG first, let dry, remove the mold, and then apply more FG while at my workbench?

Thanks for your excellent advice!

i have no idea what that other guy is talking about..with the pam and aluminum foil..but anyway...

first the foil will release from the fiberglass without the help of cooking spray..

second...fiberglassing vertically and on undersides is not impossible...it is just very difficult as the mat tends to sag...and resin drips everywhere..

best thing to do is mix your resin a little hot (by a little hot means to add a couple extra drops of hardener to the resin so it cures faster) and then apply a layer of mat..besure the peices are torn and not cut...as the frayed edges will tend to stick better then cut ones...plus the seams are less of a pian in the butt to sand...just be sure to tape and foil over everything and anything that has a chance of getting resin on it..including the route you personally will be taking to get into and out of the car...a drop sheet will work fine too...

also using a roller to squeeze out excess resin will drastically cut down on the amount of sag...if you are going to spray pam on foil i would only do it to the piece on the application side IF your going to go that routeand then roll the foil on with a roller to again squeeze out the excess resin..

only do one layer of mat at a time on vertical and upside down surfaces, to reduce wieght of the first layer...subsuquent layers will not have as much trouble sticking as the first...

regarless of the finished shape of the box you should leave it in the car as long as possible,  as fiberglass will warp and twist once removed from the forms holding it...(you car) i would not recomend the sandwich technique either.as you will only be able to do small sections at once.

like every other install if you have large flat sections you should build them out of MDF..as weight should not be your only concern..flexing sides of a box will damage your subs and sound like poop...

So you're saying that the FG won't fall off/sag? I'm considering another approach to the project- I think I might just use MDF and use some thick foam material between the underside of the rear deck and the MDF.  That will keep it from rattling.  Then I can just avoid the whole FG mess...

Thanks for your help guys.

bigpete123 wrote:

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That's great advice.  Thank you very much!  It sounds like this is quite a production, but I think it will be worth it.

Follow-up question- I know the normal process is to apply a coat of resin first, let dry for a while, apply one layer of FG, wait, apply more FG, etc etc.  If I apply 3 coats at the same time to the tinfoil/Foam/MDF apparatus and compress it against the underside of the rear dash, how will this affect drying time?  Could i maybe just do one layer of FG first, let dry, remove the mold, and then apply more FG while at my workbench?

Thanks for your excellent advice!

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The dry time will be longer but in the end it will be faster to do it 3 or more layer's at a time and this will waste less resin as well. Doing it one layer at a time is time consuming as well as you waste more resin this way.

crazyoldcougar wrote:

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i have no idea what that other guy is talking about..with the pam and aluminum foil..but anyway...

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The "other guy" has been doing this type of fiberglass work for the past 8 years and have built many systems with different techniques. I told the member Pam because it's an oil based biodegradable spray agent that you can buy anywhere. I could have steered him to PVA  but unless you know what it is or know whereto readily buy this, it's not worth mentioning. Heck, even silicone spay or Turtle Wax would work too as long as it's a barrier between the fiberglass and the tin foil.

crazyoldcougar wrote:

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first the foil will release from the fiberglass without the help of cooking spray.....

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With a release agent on there it will just come off. Without it, you will need to peel it off with your finger nails and if there are wrinkles in the foil, good luck trying to take the foil off the mold.

crazyoldcougar wrote:

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second...fiberglassing vertically and on undersides is not impossible...it is just very difficult as the mat tends to sag...and resin drips everywhere..

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This is why I suggested applying the mat & resin onto a peice of foil first & then taking this foil and appying it to a masked off area with the PAM. Using the foam & MDF board will allow the pressure to be applied to areas that have dips and rises therefore allowing you to be able to do more than one layer at a time. With the added support of the MDF, there will be no sagging and the foam will take care of the dips in the underdash area where it would normally just fall. Using something to hold up the pressure for the MDF & foam will give the fiberglass a  smooth contour to the base of the underdash.

i meant nothing bad by it man...i just didnt understand what you wrote ( how it was written)...i agreed with the pam..and if your gonna sandwich build it fine...but like i said that is only good for one panel at a time...

by the other guy i just couldnt remember who wrote it and the little window at the bottom of the screen wasnt open and i had to open a reply window..therofr not being able to scroll up and see who said it...sorry

i only said do the first layer slow...after it has cured you can definately do three at a time..plus using a roller will get rid of excess resin..

also how do you sandwich a panel that is all full of dips and crap that most folded steel and plastic panels in a car are? i have done it on flat panels before but not on multi-dimentional surfaces..

bigpete123 wrote:

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Hi.  I searched for the answer to my question and was unable to find it...  My car is a 1990 Mercedes 300ce (coupe version of the e-class).  For those unfamiliar with the early-90's E class cars, the trunk is sealed off from the cabin because the gas tank is mounted vertically just behind the rear seats.  Since the trunk is so sealed up, high fidelity bass cannot be had without some modifications.

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I also have the PITA mercedes. I have an E320 2000. I want to help you as much as I possibly can. I finally found the solution to getting very decent bass in these cars after tons of trial and error. I have built and rebuilt this thins so many times its rediculous. I would do one of these

1. this is what I had to do but it works awesome. First off you need to completely seal the box off from the trunk. I mean completely....with no holes whatsoever. I had to cut 2 big holes on the rear shelp but you may not have to depending on the output  and size of your subs. ( I have 2- 13w7's sealed)... now I built a box in the shape of a trapezoid. put the enclosure in then sealed it up to the deck with walls attached to the box. I did the sides first then the front. I guess you may need a picture to underttand what I mean.https://home.comcast.net/~martinjammin/wsb/media/809484/site1025.jpg

heres the link to all my struggles with this car 

this one has the final fix and it is something else.....https://www.carsound.com/cgi-bin/UBB_CGI/ultimatebb.cgi?ubb=get_topic&f=4&t=006872 

https://www.carsound.com/UBB/ultimatebb.php?ubb=get_topic;f=2;t=004509

https://www.carsound.com/cgi-bin/UBB_CGI/ultimatebb.cgi?ubb=get_topic&f=1&t=023689

https://www.carsound.com/UBB/ultimatebb.php?ubb=get_topic;f=4;t=006711

You have to seal the box to the rear deck. I did it with Dynamatt but there are other ways....I highly recommend you do this one....but I wouldnt cut the holes as big...It is very loud though...I mean its sick to where the rear window looks like it will break the seal and the roof flexes like crazy....this hits really hard( force to your back type feel) and  its really detailed.  Its like a half slot loaded enclosure that is corner loaded off the rear window.

the other way is to space a baffle that will butt up to the face of your sub box and has holes in it to fit the entire sub face through it. Space ti as much as you have too away from the bottom of the shelf so it clears the dips and curves and will have enough room for sub excursion.. So essentially you will have a regular sub enclosure...and a box that will fit on the face but it has an oval or something cutout for the subs.  well seal that small baffle with the holes up the rear deck with dynamat fiberglass, sprayfoam or anything that you can. I will say you need to make sure the wood needs to be either glassed permanently or give it a gasket of some sorts so the metal and wood dont touch. It will cause a nasty squeeky/rattling sound when bass hits.   But once that smaller box it attached to the rear deck all you have to do is put the box in and lift it up to the baffle with the subs through the holes and get it affixed so that it will stay there....like wedge it up so it wont break the seal.  Heres kindof and example of that smaller boxhttps://www.imagedynamicsusa.com/gallery/main.php?g2_view=core.ShowItem&g2_itemId=6593&g2_page=2 look how they do the IB setup. Now imagine its not IB and you had a sealed box butted against that baffle and sealed with the subs in the box but firing up through the holes

trust me these two methods will work and blow your mind depending on the sub...even if you build a ported box...please still seal them off from the trunk.COMPLETELY!!

or you can do IB( infinite baffle) like they do there but you will still have lots of trank rattle.

Well I guess I have to stop here because its getting kinda long. but if you have any questions let me know ...

Thanks man, I really appreciate that.  I have actually installed this before in my car.  I kept it in my original configuration for about a year, but I didn't like the way it was situated.  I basically just build a square box and cut a round hole in the metal dash.  Then I built an amp rack that fit directly under the sub box.  It wedged the box up against the underside of the dash and was simply held there by pressure.  The foam baffling around the subwoofer was pushed up against the back dash.  Eventually, that foam baffle on the sub got too compressed over time and the whole enclosure started to move around slightly.  Also, when the sub hit hard enough, it would hit the metal dash and sound terrible.

So here's my ultimate plan.  I'm not going to use fiberglass for the enclosure at all.  This will make it easier for me since I have a lot of woodworking tools already and no fiberglass experience.  It will start by cutting the hole in my dash into a square hole.  Then, I'll make a mounting plate for my sub that will fit on top of the dash (i.e. inside of the car).  It will be square with a round hole the middle.  I will seal the subwoofer to the mounting plate, and have the sub inside the car under the medkit door (that I will rebuild out of MDF so it doesn't rattle). I will dynamat (well, I'm using damplifier pro) the rear dash, and begin building the enclosure in the trunk.

I will start with the MDF piece that will butt up against the underside of the dash. I'll cut it to size and cut the hole for the sub, then I'll buy some 1-2" thick foam material from the fabric store, cut it to fit the MDF piece I make and glue it on.  This will make it so that there are no vibration problems between the metal dash and the MDF.  Then I will build the rest of the enclosure out of MDF.  I will build it such that it goes all the way to the floor of the trunk and is again held in place by pressure, like my old enclosure. The main difference will just be that my sub will be inside the cabin rather than butting up against the underside of the dash.  I'll probably get some L-brackets and some self-tapping metal screws and secure it further to the underside of the rear dash.  Somewhere in the whole mess, I'll mount my amplifier and Audiocontrol EQS 6-channel EQ...

Thanks much for your help, I appriciate it.  Any more advice?

I had a reallyinformative post that I wrote and wouldnt you know I accidentally closed the screen before sending.

basically do a round hole not square

try my setup because it is a PITA to keep the box up in the air and so it stays

this way it just sits on the floor and you can bolt it down...plus you can use plexi to display the subs front or rear

my setup has characteristics of rear firing as far as loudness but the accuracy of a forward fired enclosure.  It eliminates most mechanical sounds that you get from subs faced right at you and strenghtens the lows and tightens things up....Trust me on this...its nasty in your back.. this speakerworks s class uses the same thing  This link has more picshttps://usdaudio.com/sw/cars/mb/but here is what I'm talking about

its almost just like being in a bandpass box but without the BP sound...you get efficiency and volume but it sounds natural and has some nasty impact.

either way you go let me know so as I am putting together a tutorial for fellow Benz owners...These cars are really meant for people that dont want to modify...Well that aint me...

well Im tired of typing fro now...you should have seen the post I deleted...Ill try to get the info to you later that I lost and left out here.

cool, thanks man.  You're right, it is pretty tough to get it to stay in place.  That will also help me avoid the problem of trying to get the sub to seal (the mounting plate would somehow have to seal to the enclosure in the trunk with screws and non-hardening rope caulk.  I think I'm going to follow your advice.  Here's a link to a thread I have started on www.bnzsport.com.  My plan is to update it as I go.  You can read everything I have thought of doing for this thing in more detail that I can imagine you'd ever want!

https://www.bnzsport.com/forums/index.php?showtopic=17043&hl=pete's+audio